1. Field of the Invention
The present invention relates to a beam-splitting/coupling apparatus for splitting or coupling beams propagating through a plurality of optical fibers. More particularly, the present invention relates to a beam-splitting/coupling apparatus with a devised structure for firmly holding an optical filter in the apparatus.
2. Description of the Related Art
In recent years, in place of an electrical communication system for transmitting electrical signals through electrical wires, there has been implemented an optical communication system (an optical LAN) using optical fibers for, among other purposes, increasing the amount of transmitted information, reducing the weight of the communication system and making the construction work simple.
The optical communication system employs beam-splitting/coupling apparatuses each used for connecting a plurality of optical fibers, splitting a beam and coupling beams.
In the case of the conventional beam-splitting/coupling apparatus, an end of an optical fiber is placed on a specific side of a case having a cubic shape. An end of another optical fiber is placed on another side of the case. The other side is a side facing the specific side. An optical filter is provided inside the case, forming an angle of about 45 degrees with the optical axial line of each of the optical fibers. In addition, an end of a further optical fiber is placed on a further side of the case. This further side is orthogonal to the optical axial lines of the specific and other optical fibers.
The optical filter is attached to the case by soldering or by using an adhesive member such as an adhesive agent. The optical filter passes on a beam emitted by one of the optical fibers to another one of the optical fibers. On the other hand, a beam reflected by the optical filter enters the end of the remaining optical fiber.
As described above, in the beam-splitting/coupling apparatus, optical components of a beam emitted from an end of an optical fiber, which have wavelengths in a predetermined wavelength range, pass through an optical filter. On the other hand, optical components having wavelengths outside the predetermined wavelength range are reflected by the optical filter.
By the way, since the optical filter is an extremely small component, an excessive portion of the adhesive member used for attaching the filter to the case protrudes into the inside of an optical path, causing a fear of a negative effect on the optical performance of the optical filter.
In a process of attaching the optical filter to the case, it is feared that there is a negative effect on the optical performance if the optical filter is installed in a state of distortion. It is thus necessary to install the optical filter in the case very carefully, raising problems of a difficult assembly process which entails much time as well as labor.
It is thus an object of the present invention addressing the problems described above to provide a beam-splitting/coupling apparatus that allows an optical filter to be installed in a case of the apparatus with ease and an effect on the optical performance of the installed optical filter to be reduced.
As a first solution means for solving at least one of the problems described above, there are provided a first frame member having a contact surface as well as a beam guide path and a second frame member also having a contact surface as well as a beam guide path. An optical filter is provided between the contact surfaces with the front and back surfaces of the optical filter are held between the first and second frame members. The first and second frame members are each held inside a case by an external flexible pressure. A beam from the beam guide path of the first frame member propagates to the beam guide path of the second frame member by way of the optical filter.
As a second solution means, a pair of walls is set inside the case with the walls separated from each other. The first and second frame members are installed with outer wall portions of the first and second frame members put on between the walls.
As a third solution means, the contact surfaces of the first and second frame members are each formed in a slanting orientation relative to the direction in which one of the contact surfaces is opposite to each other. In addition, a step portion is formed on each of the contact surfaces. Mutually facing side-edge surfaces of the optical filter are held between the step portions under external flexible pressures.
As a fourth solution means, a protrusion is formed on at least one of the contact surfaces of the first and second frame members. A low-edge surface of the optical filter is mounted on the protrusion.